Sea surface antenna

Abstract

A sea surface antenna comprises a cylindrical tube of metallic material 13 on a dielectric former. The tube has a longitudinal slot shorted at each end and coupled at its midpoint to a coaxial feed. The slot is bridged by two groups of capacitances each group being distributed along a respective half of the slot. The length of the antenna is less than 0.25 λ and the diameter of the antenna is less than 0.02 λ, where λ is the free space wavelength at the operating frequency. The antenna is dimensioned so as to operate in an evanescent mode at a resonant frequency less than the cut-off frequency.

1. A sea surface antenna comprising a tube of metallic material, the tube having a substantially longitudinal slot coupled at its midpoint to a feed line, the slot being bridged by two pluralities of varactor diodes to either side of the feed-point, each plurality being distributed along a respective part of the slot, the length of the antenna being less than 0.25 λ, where λ is the free space wavelength at the operating frequency, the antenna being dimensioned so as to operate in an evanescent mode at a resonant frequency less than the cut-off frequency, the antenna being provided with means for applying a variable bias to the varactor diodes. 
   
   
     2. An antenna according to  claim 1  wherein the slot is shorted at each end. 
   
   
     3. A sea surface antenna comprising a tube of metallic material on a dielectric former, the tube having a longitudinal slot coupled at its midpoint to a feed line, the slot being bridged by two pluralities of varactor diodes to either side of the feed-point, each plurality being distributed along a respective part of the slot, the length of the antenna being less than 0.25 λ and the diameter of the antenna being less than 0.02 λ, where λ is the free space wavelength at the operating frequency, the antenna being dimensioned so as to operate in an evanescent mode at a resonant frequency less than the cut-off frequency, the antenna being provided with means for applying a variable bias to the varactor diodes. 
   
   
     4. An antenna according to  claim 3  wherein the slot is shorted at each end. 
   
   
     5. A sea surface antenna arrangement including two or more like antennas placed in a colinear configuration and connected electrically in parallel, wherein each of said like antennas comprises a tube of metallic material on a dielectric former, the tube having a substantially longitudinal slot coupled at its midpoint to a feed line, the slot being bridged by two pluralities of varactor diodes to either side of the feed-point, each plurality being distributed along a respective part of the slot, the length of the antenna being less than 0.25 λ and the diameter of the antenna being less than 0.02 λ, where λ is the free space wavelength at the operating frequency, the antenna being dimensioned so as to operate in an evanescent mode at a resonant frequency less than the cut-off frequency, the antenna being provided with means for applying a variable bias to the varactor diodes.